This invention relates to fault detection in electrical power distribution systems. It finds particular application to the detection and notification of subtle feeder faults in overhead and underground feeders that are not ordinarily detected by conventional feeder protection intelligent electronic device (IEDs).
Underground and overhead feeders and cables are a key component in the transmission and distribution of electrical power. Unfortunately, however, overhead feeders and cables can be prone to short circuits or otherwise abnormally low impedance connections between two or more phases or between one or more phases and ground. These and other feeder faults including cable faults can be caused by a number of factors, including human error (e.g., accidentally cutting or striking a cable), climatologic conditions (e.g., precipitation, seismic activity, or lightning strikes), animal activity, and failure or degradation of the cable or its associated equipment due to the aging of the insulation. Moreover, feeder faults including cable faults can lead to power outages, which are inconvenient for the affected customers and which can be expensive for the electric utility involved.
One category of feeder faults is that of self-clearing faults. While self-clearing faults can have any number of root causes, they typically have a temporal duration which is insufficient to trip the relevant protective device. In practice, the duration of most self-clearing faults is typically less than about one (1) to two (2) cycles of the power system frequency, and in many cases less than one (1) cycle.
One mechanism which can generate self-clearing cable faults is a temporary breakdown in the insulation between cable phases or between a cable phase and ground. Such faults are often caused or exacerbated by moisture at a cable splice or joint, and are typically characterized by an elevated or fault current having a duration of about one-quarter to one-half cycle (i.e., roughly four (4) to eight (8) milliseconds (ms) in a sixty Hertz (60 Hz) system). The onset of the fault current usually occurs at or near a voltage peak where the electric field stress is the highest. As the situation deteriorates, the frequency and severity of these faults tend to worsen with time, culminating in an eventual cable failure and a resultant power outage.
Detection apparatus has been developed to identify self-clearing faults. For example, see (i.) Kojovic, et al., Sub-Cycle Overcurrent Protection for Self-Clearing Faults Due to Insulation Breakdown (1999); and (ii.) U.S. Pat. No. 6,198,401 to Newton, et al., Detection of Sub-Cycle, Self-Clearing Faults, issued Mar. 6, 2001. In prior art fault detection devices such as those identified above, a transient fault is automatically determined to be a self-clearing fault of the type associated with cable degradation. However, the inventors have determined that such transient faults are not necessarily self-clearing faults attributable to cable degradation. In some instances, such transient faults may be attributable to arc propagation from another cable or from environmental or other factors that affect more than one cable.
Furthermore, in conventional devices, the user has to set the program settings manually and appropriately for the algorithm to work properly. Setting the parameters requires a-priori knowledge of the system parameters, load currents, as well as fault current levels. The issue may be resolved when the settings are entered for the first time but fault currents and circuit loadings are subject to change on a frequent basis due to manual or automated switching and reconfiguration of the circuits. When the circuit is reconfigured, the algorithm settings need to change accordingly to reflect the latest status of the circuit. Changing the parameters on-the-fly and automatically has not been a common practice to date. By making use of adaptive settings, the algorithm adapts dynamically to the changing environment and prepares itself for correct operation without user intervention.
Finally, conventional fault detection functions typically reside at the level of the protective relay with fixed implementation. One consequence of this relay-centric architecture is that the apparatus is relatively poorly integrated with the substation automation (SA), distribution automation (DA), feeder automation (FA), or other automation system. Moreover, the apparatus requires the use of a specialized hardware platform which must be closely coupled to the protective relay. Furthermore, the detection techniques have been relatively unsophisticated and make use of instantaneous fault currents. Although this minimizes the detection delay, without proper filtering, the techniques may be prone to noise and outliers resulting in nuisance false alarms.
The present invention is directed toward methods and apparatus that address the foregoing deficiencies of prior art fault detection methods and apparatus.